The application of coating materials using electrostatic spraying techniques has been practiced in industry for many years. In these applications, the coating material is discharged in atomized form and an electrostatic charge is imparted to the atomized particles which are then directed toward a substrate maintained at a different potential to establish an electrostatic attraction for the charged, atomized particles. In the past, coating materials of the solvent-based variety, such as varnishes, lacquers, enamels and the like, were the primary materials employed in electrostatic coating applications. The problem with such coating materials is that they create an atmosphere which is both explosive and toxic. The explosive nature of the environment presents a safety hazard should a spark inadvertently be generated, such as by accidentally grounding the nozzle of the spray gun, which can ignite the solvent in the atmosphere causing an explosion. The toxic nature of the workplace atmosphere created by solvent coating materials can be a health hazard should an employee inhale solvent vapors.
As a result of the problems with solvent-based coatings, the recent trend has been to switch to waterbased coatings which reduce the problems of explosiveness and toxicity. Unfortunately, this switch to water-based type coatings has sharply increased the risk of electrical shock, which risk was relatively minor with solvent-based coatings. The problem of electrical shock has been addressed in U.S. Pat. Nos. 5,078,168 and 5,197,676, both owned by the assignee of this invention. In systems of the type disclosed in these patents, a "voltage block", i.e. an air gap, is provided between one or more sources of the conductive coating material and the electrostatically charged coating material which is directed to the coating dispensers. This voltage block insures that there is never an electrical path between the source of water-based coating material and the high voltage electrostatic power supply.
In systems of the type disclosed in U.S. Pat. Nos. 5,078,168 and 5,197,676, a voltage block is formed by operation of a first shuttle device connected to the reservoir of a first piston pump, and a second shuttle device connected to the reservoir of a second piston pump. The first shuttle is movable with respect to a filling station, which is connected to one or more sources of water-based paint, between a transfer position coupled to the filling station and a neutral position physically spaced or separated by an air gap from the filling station. The second shuttle is movable with respect to a discharge station, which is connected to the reservoir of the first piston pump, between a transfer position coupled to the discharge station and a neutral position spaced from the discharge station. The reservoir of the second piston pump, connected to the second shuttle as noted above, communicates through a feed line with a number of spray guns. Movement of the first and second shuttle devices between their respective transfer and neutral positions is controlled such that when one of the shuttles is in a transfer position the other is in a neutral position to ensure that a voltage block or air gap is constantly maintained at some point along the path from the source(s) of coating material to the coating dispensers. In alternative embodiments of systems of the type disclosed in U.S. Pat. Nos. 5,078,168 and 5,197,676, the second shuttle device and second piston pump can be eliminated in which case the first piston pump is connected directly to one or more manually operated spray guns, and operation of a single shuttle device is controlled to maintain a voltage block between the paint source and spray guns(s).
One potential limitation of voltage block systems of the type disclosed in U.S. Pat. Nos. 5,078,168 and 5,197,676 is that the control system for moving the first shuttle and/or second shuttle between the transfer position and the neutral position includes electrically operated valves, switches and other electrical components. While the shuttles and pumps are driven by pneumatic actuators, the operation of such actuators is nevertheless controlled by electrical valves, switches and the like. Because of the highly conductive nature of water-based coating materials, it is preferable to eliminate, or at least reduce, the amount of system control accomplished by electrical components. Additionally, the control of pneumatic actuators by electrical components complicates the control system, requires special wiring upon installation of the equipment at the customer's facility, and, adds expense both for initial installation and subsequent maintenance.